In mobile radio communications, the radio spectrum is a scarce resource. As a result most mobile radio communications system are based on the cellular principle. Basically, a geographical area, within which wireless service is available, is divided into several cells. Schematically, each cell is represented as a hexagon; in practice, however, each cell has a shape that is dependent on, among other things, the topography of the terrain serviced by the system. Each cell includes a base station, which may be located approximately at its center. Each base station is configured to transmit and receive signals within approximately the area defined by each cell. However, the actual radio range of each base station may extend beyond each cell area. Therefore, it is desired that a different set of frequencies be allocated to the adjacent cells to avoid interference. Subscribers located within each cell area communicate with other subscribers by using a wireless terminal (e.g., a cellular telephone, a wireless local loop terminal, some cordless telephones, one-way and two-way pagers, PCS terminals and personal digital assistants). Each wireless terminal located within a cell sends to and receives signals from the corresponding base station located in that cell, over a communications channel within a predetermined frequency range.
Since adjacent cells employ different sets of frequencies, the distance between two cells that use the same frequency set may be an important design consideration. This distance is called the mean reuse distance D. In order to increase the total number of channels available per unit area, it is desired to decrease the size of the cells. By reducing the size of the cells, it is possible to reuse the same frequency sets more often. Thus, more subscribers may be able to use the system, because of the increase of available frequency sets within a predetermined area. However, depending upon the size of each cell, the transmission power of the base stations and the mobile units, severe co-channel interference between the cells that use the same frequency range may occur.
The maximum likelihood sequential estimation (MLSE) equalizer can equalize the channel at the receiver to achieve optimal performance. The MLSE equalizer is particularly useful in a radio channel with a long spread such as the one that employs a standard specification known as Global System for Mobile Communications (GSM). Another approach to reduce co-channel interference is to employ antenna arrays. Because of the frequent spatial separation between the desired signal and the co-channel interference signals, antenna arrays can suppress co-channel interference signals through beam forming. However, these approaches require very complicated signal processing in order to produce optimal results.
Thus, there is a need to reduce the complexity of such systems, to make them commercially feasible, and to reduce the effects of co-channel interference signals considerably.